The most effective way to tell if a ring is real gold is to take it to a jeweler for inspection. There are also some simple tests that can be done at home.
Inspect the ring for a symbol stamped into the gold. Real gold ranges from 10K to 24K, with 24K being the highest gold content and the most desirable. However, some fake gold is stamped with these symbols.
Check to make sure the ring has no discoloration or dark patches. Real gold does not rust, while fake gold and gold-plated items eventually wear down and reveal their true nature.
It is possible to test gold with nitric acid. Real gold has no reaction to the acid, while fake products change color depending on their metal content.